A rolling bearing composed mainly of races (inner race and outer race) and rolling elements is used in a severe way that a shear stress is repeatedly imposed under a high plane pressure so that a sufficient rolling fatigue life to stand the shear stress must be assured. Besides the rolling stress, a slip repeatedly occurs during running between the races and the rolling elements to cause wear so that a bearing is also required to have satisfactory wear resistance.
The demands for fatigue characteristics and wear resistance of a rolling bearing have generally been met by using high-chrome carbon steel as a bearing material and subjecting the steel to hardening (quenching) and tempering or carburizing case hardening steel followed by hardening and tempering.
In recent years, however, machinery using rolling bearings has been increasing the speed of running and the load applied thereon, and accordingly, the conditions of using bearings have been getting severer, such as an increased temperature, insufficient oil film formation, and a high PV value, now making the wear resistance obtainable from conventional bearings unsatisfactory.
To cope with such a severe environment of usage, wear resistance has been improved by using high alloy steel having a high content of carbide-forming elements, such as stainless steel and high speed steel (e.g., SKH series or M50 steel), as a bearing material, which is subjected to precipitation hardening by precipitating a large amount of carbides in the surface layer to give an increased surface hardness.
Problem to be Solved by the Invention:
Bearings made of high carbon bearing steel (SUJ2) or general carburized case hardening steel have a short bearing life, being worn considerably rapidly under the above-mentioned severe conditions.
Bearings made of the above-described high alloy steel (e.g., stainless steel or high speed steel) show improved wear resistance compared with those made of conventional bearing materials, such as high carbon steel (SUJ2) or general carburized case hardening steel.
In a severer environment, however, even those bearings made of the high alloy steel undergo wear in a band form on the raceways and on the rolling surface of the rolling element as shown in FIG. 16(a), an enlarged photograph of the surface of a rolling element, in which the slightly black vertical band is a worn part. That is, there is a fear that the trace of running of a rolling element on the raceways becomes fixed to locally wear the bearing, causing vibrations and deterioration in acoustic characteristics, leading to surface damages such as peeling, and breaking the bearing.
This is partly because there is no correlation between the surface hardness at room temperature and wear resistance as hereinafter described. That is, an improvement in surface hardness at room temperature does not always lead to an improvement in wear resistance (see FIG. 3).
The present invention has been reached with attention paid to the above-described problems. An object of the present invention is to provide a rolling bearing having sufficient wear resistance for use under severe conditions to secure an extended bearing life.